Thermionic valve circuit arrangement



July 4 1939 G. M. WRIGHT 64,559

THERMIONIC VALVE CIRCUIT ARRANGEMENT Filed Feb. 1, 19:57

4 INVENTOR G50/eas MAUR/tf WR/G/l ATTORNEY Patented `uly 4, 193g UNHTE STS OFFICE THERMIONIC VALVE CKRCUIT ARRANGE- MEN Application February 1, 1937, Serial No. 123,318 In Great Britain February 8, 1936 (Cl. Z50-36) 8 Claims.

This invention relates to thermionic Valve circuit arrangements and more particularly to circuit arrangements employing thermionic valves of the type now known as electron beam valves, i. e., of the type wherein the electron discharge is in the form of a jet or beam rather than in the form of a stream and partakes more of the nature oi the discharge in a cathode ray tube than of that in an ordinary thermionic valve as at present in common use. Electron beam Valves are described in the British Patent No. 403,973 among others.

The main object of the present invention is to provide an improved electron beam valve circuit arrangement wherein controllable reaction may be obtained. The obtaining of controllable reaction is often required, for example in radio receiving sets, for the purpose of increasing selectivity and/or increasing amplification.

The invention is illustrated in and explained in connection with the accompanying diagran matic and graphical drawing in which Fig. l shows a conventional circuit as an aid to eX- plaining the invention; Fig. 2 shows graphically the relationship between amplification and accelerator voltage of a cathode beam tube; and Fig. 3 shows one embodiment of the invention.

If an electron beam valve be connected in an ordinary back coupled reaction circuit certain diiculties which will be described later herein arise. Consider, first the circuit shown in Figure l which is an explanatory figure and represents a circuit which is not in accordance with the present invention. Referring to Figure l, the target or collector electrode I of an electron beam valve 2 is connected through the usual output parallel tuned circuit 3 in series with a source l of anode voltage to the cathode 5 and the control electrode 6 is connected to the cathode 5 through a reaction coil 'l (which is coupled tothe coil in the output circuit 3) in series with a bias battery 8. The guard electrode 9 of the valve 2 is connected to the cathode 5 directly (the guard electrode 9 is provided for the purposes of minimizing secondary emission eiTects and providing screening) and the accelerator anode Il) is connected to a suitable point Il upon the anode potential source li. This circuit arrangement constitutes the ordinary well known inductive feed back reaction circuit applied to an electron beam valve. If in this circuit arrangement the coupling between the reaction coil 7 and the coil in the output circuit 3 be Varied it will be found that smooth and gradual control of the reaction (such as is obtainable with this circuit when employed with an ordinary Valve) up t0 a point where the circuit reaches selfoscillation is not readily obtainable but that adjustment of the coupling is very critical (especially when near the self-oscillatory condition) so that in practice it is almost impossible to bring the circuit to a point near the self-oscillatory condition with reasonable certainty that the circuit will not suddenly burst into self-oscillation. lThe reason for this difliculty is to be found in the characteristics of the electron beam valve as distinot from the characteristics of an ordinary valve. In the first place the impedance of the target or collector electrode circuit of an electron beam valve is very high-being usually many inegohms-and the target-gun space oi the valve imposes no appreciable damping on the tuned circuit in the target circuit. In consequence, therefore, as the coupling between the reaction coil and the output coil increases, a point is reached when the whole arrangement suddenly bursts into self-oscillation of such a magnitude that the control electrode becomes positive to the cathode at the limit of its swing. Accordingly current flows in the control electrode circuit and the presence of this current damps the increasing oscillation in the output tuned circuit via the coupling between said tuned circuit and the reaction coil, and a steady state finally results wherein substantially constant oscillatory current iiows in the output tuned circuit, Now, as is well known, one of the 30 most important uses of reaction in a receiving circuit is ior the reduction of the effective damp'- ing of an oscillatory circuit to a very low value.

If with an electron beam valve connected in the known reaction circuit of Figure l, such reduction :i3 of eiective damping was sought to be obtained by increasing the coupling between the reaction coil and the output coil, a point would soon be reached where quite a small chance disturbance or tran sient would be sufficient to cause the whole ario rangement to burst into self-oscillation, the oscillations building up with great rapidity to the limiting value already referred to.

With a normal triode in the known reaction circuit oi Figure l the ordinary curvature of characteristio is sufficient to avoid this critical or unstable condition, for the curvature of the anode characteristic of an ordinary triode is such that the presence of an oscillation slightly reduces the over-all magnification of the valve. The result is that when using the known reaction circuit of Figure 1 with an ordinary Valve it is possible by careful adjustment of the coupling between the reaction coil and the output coil to maintain weak oscillations of desired amplitude in the output 55 circuit and indeed to keep the whole arrangement in a condition of "very nearly zero effective resistance by suitable critical adjustment of the reaction coupling and of the applied voltages. Where, however, an electron beam valve is used in the same circuit arrangement, low effective damping is very difficult, if notr impossible, to secure, for the target characteristic of such a valve is a substantially straight line parallel to the target voltage axis of the curve.

'I'he present invention avoids the difficulties above referred to and enables an electron beam valve to be operated in a sensitive and controllable reaction circuit to give controllable reaction up to the point of self-oscillation and enables oscillations of controllable magnitude-including quite weak magnitudes-to be generated.

According to this invention a reaction circuit arrangement employing an electron beam valve is characterized in that a component of oscillatory voltage from the output circuit of the valve is fed back to the accelerator electrode of the valve.

Figure 2 is a graph connecting the output current (ordinates) of an electron beam valve operated without reaction and the accelerator electrode voltage (abscissae) and it will be seen that the said curve has a fairly well defined though lnot sharply peaked maximum value at P. The present invention makes use of this propertylof electron beam valves to enable smoothly controllable reaction to be obtained.

Figure 3 shows one way of carrying out this invention. Referring to Figure 3 an electron beam valve 2 has an output tuned circuit 3 connected at one' end to the target I and at the other end through a suitable coupling condenser I2 to the accelerator anode IB. A tapping point I3 upon the coil inthe output tuned circuit 3 is connected through a source l of target potential to the cathode 5 and a tapping point I I upon this source is connected through a high frequency choke I4 to the accelerator anode Ill. The guard electrode 9 is directly connected to the cathode 5 and the control electrode 6 is connected through a reaction coil 'I in series with a bias source 8 to the cathode 5. The reaction coil may be adjustably coupled to the output coil as in the usual way.

As a preliminary suppose that the tapping I3 upon the output coil is central. This tapping is maintained at a steady potential by the target potential source 4 and since the accelerator anode I0 is capacity coupled through condenser I2 to one end of the output coil, the saidV accelerator anode I0 will'swing in voltage by half the total potential swing across the output'tuned circuit 3. If the potential applied to the accelerator anode IQ has been set to the optimum voltage (as given by the curve of Figure 2) the amplification of the val-ve will swing to and fro about the maximum, and its mean value will be less than if the potential on anode IB were maintained steady at the optimum value, the exact amount of the reduction of magnification depending upon the amplitude of the oscillations and of the shape of the curve above referred to.

It will be apparent therefore that stable and smooth reaction conditions will be obtained for immediately any oscillation is set up in the output circuit 3 the oscillation build up will be controlled by the eiective reduction of amplication of the valve. The intensity of this control may be adjusted by varying the position of the tapping t3 upon the output coil, the nearer this tap is to the acceerator anode and of the output coil the less being the said control.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. A reaction circuit arrangement comprising an electron beam Valve having a cathode, a con- Y trol electrode, an accelerating and focusing electrode for forming electrons from the cathode into av beam, and a target electrode, an output circuit connected at one end to the target electrode of said valve and at the other end through a condenser to the accelerating and focusing electrode of' saidv valve, a connection including a source of target potential, between the cathode of said valve and an intermediate point on said output circuit,r a connection including a reaction coil and a bias potential source between the control electrode of said valve and said cathode, said reaction coil being coupled to said output circuit, and a connection including a choke between said accelerating and focusing electrode and an intermediate tap on said source of target potential whereby energy fed from the output circuit to said accelerating and focusing electrode reduces the effective amplication factor of the tube.

2. An arrangement as claimed in claim l wherein the intermediate point on the output circuit is adjustable.

3. In an oscillating circuit wherein is provided an electron beam tube having a cathode, control electrode, an accelerating and focusingv electrode, a target electrode, and a resonant circuit, the method of improving the control of oscillations which comprises the steps of focusing electrons from the cathode into a beam, projecting the beam toward the target electrode to produce oscillations in the resonant circuit, and feeding back energy from the resonant circuit to the accelerating and focusing electrode in anti-phase relation whereby the eiective amplification of the tube is reduced.

4. An oscillator comprising an electron beam tube having a cathode, an accelerating and focusing electrode, and a target electrode, a tuned circuit connected to the target electrode, a source of energy connected between the cathode and the tuned circuit, and a connectionirom the tuned circuit to the accelerating and focusing electrode to vary the accelerating electrode potential in opposite phase to variation of the target electrode potential whereby the effective amplification of the tube is reduced.

5. An oscillator comprising an electron beam tube having a cathode, an accelerating and focusing electrode and a target electrode, a circuit including an inductance, a connection from one end of the inductance to the target electrode, a connection from the other end of the inductance to the accelerating. electrode to reduce the eiiective amplication of the tube, and a source of energy connected between the cathode and the inductance at 4a point intermediate the two ends of the inductance.

6. Themethod of controlling the ampliiication of an electron beam tube having a cathode, an accelerating anc] focusing electrode andV a target, which includes the step of' supplying the Vaccelerator electrode and the target electrode with a predetermined potential, and altering the accelerating and focusing electrode potential in proportion to time in. the opposite sense of variations of potential of the target electrode to reduce the eiective amplication of the tube.

7. An oscillator comprising an electron beam tube having a catho-de, an accelerating and focusing electrode and a target electrode, a network connected Ibetween the target electrode and the cathode, and an impedance connection connected from the network to the accelerating electrode to Vary the voltage of the accelerating electrode in opposite phase to the target electrode voltage variation whereby the effective amplification of the tube is reduced.

8. An oscillator comprising an electron beam tube having a cathode, an accelerating and focusing electrode and a target electrode, a network connected. between the target electrode and the cathode, and impedance feedback means connected between the network and the accelerating electrode to control the beam by varying the voltage of the accelerating electrode in opposite phase to the target electrode voltage variation.

GEORGE MAURICE WRIGHT. 

